1. Field
One embodiment relates to an optical element, an optical element production method, and a light source device provided with the optical element.
2. Description of the Related Art
There is a known method making use of the second-order nonlinear optical effect of a nonlinear optical crystal to generate a second harmonic which is light of a wavelength equal to half that of incoming light into a solid-state laser (Second harmonic generation; SHG) and to use it as a short-wavelength laser beam. The nonlinear optical effect is an effect by nonlinearity of polarization response of substance, which is a phenomenon in which when strong light such as a laser beam is injected into a substance, the polarization response becomes non-proportional to an electric field of incoming light whereby part of the incoming light is subjected to wavelength conversion.
The second harmonic generated in fact has the wavelength slightly shorter than half of the wavelength of the incoming light because of the dispersion property of crystal. For this reason, phases of second harmonics become gradually deviating from each other, so as to result in cancellation of second harmonics with each other. In this case, it becomes difficult to generate the second harmonic in sufficient quantity.
For obtaining the second harmonic in sufficient quantity, there is a Quasi-Phase Matching element (QPM element) proposed. Quasi-phase matching is a method to avoid the cancellation of electric fields of second harmonics, in which a polarity inversion structure is formed in a crystal to change polarities into such directions as to avoid the cancellation of electric fields, thereby achieving quasi-phase matching to enable second harmonic generation. Concerning the quasi-phase matching element, recent investigation has been directed, particularly, toward a quasi-phase matching element in which a periodic polarity inversion structure is formed by applying an external force to quartz crystal as a paraelectric material (e.g., cf. Japanese Patent Application Laid-Open No. 2004-107187 and Japanese Patent Application Laid-Open No. 2004-279612).
To realize the quasi-phase matching, it is necessary to obtain a structure in which polarities of crystal are periodically inverted. For short-wavelength generation such as ultraviolet light, an ordinary method employed is to form polarity-inverted domains inside a single crystal. When the polarity inversion structure is formed in a ferroelectric material, a method employed is to invert spontaneous polarization by applying an external electric field. However, for example, in the case of ferroelectric materials such as lithium niobate and tantalum niobate, the absorption edge wavelength to determine the generated wavelength of the second harmonic is long, about 300 nm. For this reason, it becomes impossible to implement the second harmonic generation in the range of not more than 200 nm called the vacuum ultraviolet region. On the other hand, for example, quartz crystal as a paraelectric material is a nonlinear optical crystal with the absorption edge wavelength of not more than 150 nm.